1. Technical Field
The present invention relates to a machining unit and a machine tool that machine a workpiece by a revolving tool with a main spindle section being driven and rotated.
2. Related Art
Conventionally, for machining workpieces, a machine tool that is suitable for a type of the machining has been used. For example, when an outer circumferential surface of a cylindrical pipe is to be threaded, a lathe is used. The threading is performed in this lathe as follows: the pipe rotates about its axis in an integrated manner with a main spindle section of the lathe while the pipe being fastened by a chuck onto the main spindle section, and a tool moves straight in the axis direction of the pipe with a cutting edge of the tool abutting against the outer circumferential surface of the pipe.
However, in the case of a pipe whose diameter or length is large, more power is needed to rotate the pipe and a large lathe is needed; this results in the increase of costs. Besides, if the size of a pipe is large, the self-excited vibration is likely to occur as the pipe rotates, and this makes it difficult to maintain the machining accuracy.
As a method for solving these problems, there is a method referred to as U-axis machining, in which a machining center is used. The U-axis machining is a method in which, unlike lathes, a workpiece is immovably fastened and machined by revolving a tool while adjusting the radius of the revolution of the tool.
That is, as shown in the side views of FIGS. 1A and 1B, a pipe 101 is fastened immovably at a certain position. A machining center includes a main spindle section 121 that is driven and rotates, and the main spindle section 121 is provided with a tool 143 whose position is adjustable radially with respect to a rotation center C121. After adjusting the revolution radius of the tool 143 in such a manner as the tool 143 is located at a position corresponding to the diameter of the pipe 101 as shown in FIG. 1B, the main spindle section 121 rotates with the rotation center C121 of the main spindle section 121 being aligned with an axis C101 of the pipe 101, as shown in FIG. 1A. Thereby, a cutting edge of the tool 143 revolves along an outer circumferential surface of the pipe 101 and, in conjunction therewith, the main spindle section 121 moves in the axis direction of the pipe 101, so that helical threads are formed on the outer circumferential surface of the pipe 101.
As a disclosed art relating to this type of U-axis machining, there is an apparatus disclosed in Japanese Examined Patent Publication No. 62-42726, for example. That is, as shown in FIGS. 2A and 2B, which are views of FIG. 1B along arrows II, the main spindle section 121 of the apparatus is provided with an eccentric rotational section 131 to rotate about an eccentric axis C131 that is located eccentrically in the radial direction from the rotation center C121; the tool 143 is fastened to the eccentric rotational section 131. Therefore, by rotating the eccentric rotational section 131 from a state shown in FIG. 2A to a state shown in FIG. 2B, the tool 143 moves in the radial direction of the main spindle section 121; as a result thereof, it is possible to set the revolution radius of the tool 143 to a radius corresponding to the diameter of the pipe.
In terms of the cutting capability during machining, it is desirable to turn a cutting edge 143E of the tool 143 toward the rotation center C121 of the main spindle section 121. However, in the above-mentioned apparatus, since the tool 143 is fastened on the eccentric rotational section 131 in such a manner as the tool 143 cannot rotate, it is impossible to adjust the orientation of the cutting edge 143E of the tool 143. Therefore, even if, as shown in FIG. 2A, the cutting edge 143E properly faces the rotation center C121 of the main spindle section 121 when the revolution radius of the tool 143 is a certain value, the cutting edge 143E does not face the rotation center C121 when changing the revolution radius, as shown in FIG. 2B. As a result thereof, in the apparatus, it is impossible to maintain a good cutting capability for various pipe diameters.
At the end of screw-threading process, the revolution radius of the tool 143 is changing continuously during screw-threading, in such a manner as the thread becomes gradually shallower. In this case, for the same reason as mentioned above, there is a risk that the deterioration of machining accuracy would be caused by the deterioration in cutting capability as a result that the orientation of the cutting edge 143E of the tool 143 turns away from the direction of the rotation center C121 of the main spindle section 121 at the end of screw-threading process.